1. Field of the Invention
The present invention relates to a method for forming a thin film and, more particularly, to a method for forming an insulating film having a low dielectric constant, which is suitable for intermetal insulating film applications, by plasma enhanced chemical vapor deposition (PECVD).
2. Discussion of Related Art
With the reduction of the dielectric constant of a thin film in an intermetal insulating film application, the operating speed of a device can be increased in a smaller sized device, and the cross-talk can be decreased. As a result, power consumption can be reduced. To achieve these advantages, resulting from such a reduction of the dielectric constant, various methods for lowering the dielectric constant of an intermetal insulating film have been suggested.
The dielectric constant of a conventional intermetal insulating film (IMD) of silicon oxide (SiO.sub.2) is about 4. However, an insulating film of silicon oxide (SiO.sub.2) has a disadvantage in that its dielectric constant increases to 10 when the film is exposed to moisture.
An example of a conventional low dielectric constant insulating film is a silicon oxide film containing fluorine.
The dielectric constant of this fluorine-containing silicon oxide (F.sub.x SiO.sub.y) film is 3.about.3.7. In U.S. Pat. No. 5,334,552 to Homa et al. Homa et al. disclose a method for forming an insulating film having a multi-layered interconnection structure which includes the step of forming a 2 to 3.5 .mu.m thick fluorine-containing silicon oxide film at a temperature of not higher than 200.degree. C. Another example is U.S. Pat. No. 5,429,995 to Nishiyame et al., in which Nishiyame et al. disclose a method for depositing a low dielectric constant and low hygroscopic fluorine by a plasma chemical vapor deposition method using a source gas containing silicon, oxygen and fluorine.
Further examples of conventional low dielectric constant insulating films are organic polymers having a low polarity molecular structure. Spin-coatable polyimides have dielectric constants in the range of 3.0 to 3.7. For instance, U.S. Pat. No. 5,428,102 discloses that a series of high temperature, low dielectric constant, aromatic polyimides have been developed. One of the challenges when using organic polymers is temperature stability. In current processes involving low dielectric constant insulating films, CVD tungsten is deposited at around 450.degree. C., and annealing is performed at around 400.degree. C. Most of the low dielectric organic polymers, however, cannot withstand such high temperature processing.
Among various organic polymers, fluoropolymers such as polytetrafluoroethylene (PTFE) have by far the lowest dielectric constant. For instance, amorphous PTFE has a dielectric constant of 1.9. However, their low adhesive force, low thermal stability and difficulty of manufacture have hindered their use in microelectronics.
On the other hand, hydrogenated amorphous carbon (a-C:F) films deposited by plasma deposition from hydrocarbon gases have high electric resistivity, good thermal stability due to their highly cross-linked structures, and are easy to manufacture. Endo and Tatsumi (1995), J. Appl. Phys. 78(2), 1955, pp 1370, proposed fluorinated amorphous carbon (a-C:F) thin films, which have both a cross-linking and a PTFE-like structure, as low dielectric constant interlayer dielectrics for ultra large scale integration (ULSI) multilevel interconnections. When the source gas was 94% CF.sub.4 and 6% CH.sub.4, the thin film on the powered electrode showed a dielectric constant of 2.1. The film showed high stress and required an adhesion layer of 10 nm thick a-C:H. The film shrank to 75% when annealed at 300.degree. C. The leakage current was 10.sup.-7 A/cm.sup.2 at 1 MV/cm, and was suppressed to 10.sup.-8 by fluorination.
Another example of a conventional low-dielectric constant, amorphous fluorocarbon film for use as an intermetallic dielectric is U.S. Pat. No. 5,302,420. This patent discloses a plasma-deposited polymeric fluorocarbon film with a dielectric constant of about 2.5 and with a thermal stability of at least 350.degree. C. The film is deposited in an asymmetric electrode type reactor at a pressure in the range of 10 to 180 mTorr and with a self-bias voltage in the range of -50 to -700V. Films with a thickness between 0.05 to 5 .mu.m could be deposited. The hardness and the thermal stability of the amorphous fluorocarbon film stems from the high bombardment used during the deposition process.
An example of a conventional low dielectric plasma-polymerized fluoropolymer thin film is U.S. Pat. No. 4,938,995. This patent discloses a process for the deposition of a low dielectric constant(in the range of 2.3 to 3.3) fluoropolymer thin film using oxygen-containing fluoropolymers as the source monomer.
Another example of a conventional low dielectric constant insulating film is a fluorinated diamond-like carbon film disclosed in U.S. Pat. No. 5,462,784. This patent discloses an improved wear-resistant protective coating for the surface of recording devices that is formed of fluorinated diamond-like carbon. The films are prepared by plasma enhanced chemical vapor deposition on a negatively biased substrate from mixtures of fluorinated hydrocarbons with hydrogen; preferably from fluorinated hydrocarbons with a large ratio of fluorine to carbon in the molecule such as hexafluorobenzene (C.sub.6 H.sub.6) and pentafluorobenzene (C.sub.6 HF.sub.5).
Conventional low dielectric constant insulating films have the following problems. First, the films are not stable at temperatures exceeding 350.degree. C. Second, the films deposited using high ion bombardment have severe internal stress which makes them unsuitable for device fabrications. For example, variations of fluorocarbon moieties were obtained at temperatures ranging from 20.about.700.degree. C. The fluorocarbon films consisting of CF.sub.3, CF.sub.2, CF and C--CF.sub.x bonds were found to be stable up to approximately 200.degree. C. and to be pyrolyzed via dissociation of the thermal bonds above this temperature.